The present invention relates generally to the control of electromagnetic interference from electronic devices and more particularly to the control of electromagnetic interference generated or present in the region of an integrated circuit.
Electromagnetic interference (EMI) is a generic term for unwanted interference energies either conducted as currents or radiated as electromagnetic fields. EMI can emanate from electronic devices in several ways. Generally, voltages and currents in board mounted, integrated circuits create electric and magnetic fields that radiate from the device. EMI radiating from such devices will vary in field strength and impedance according to the shape and orientation of the conductors, the distance from the conductors to any shielding provided by circuit components or by coupling to circuit components.
Since many electronic devices and integrated circuits operate by generating signals at rates in excess of a million Hertz, radiating EMI will extend into the radio frequency spectrum and can cause significant interference with radio and television signals. The United States Federal Communications Commission has promulgated rules and procedures requiring the minimization and regulation of EMI by manufacturers of electronic devices. Under such regulations, EMI must be limited for certain devices to particular radiated power ranges depending on the frequency of the EMI signal. In lieu of the problems surrounding ESI and the need to comply with government regulations, various schemes have been proposed for limiting EMI. One typical scheme has been to provide a conductive enclosure to an electronic device so that EMI field lines will terminate on such enclosure. Unfortunately, conductive enclosures which contain the entire product or parts of the product can be very expensive. Furthermore, these conductive enclosures are typically designed concurrently with the electronic device so that they if they need to be redesigned due to EMI problems it causes additional expense and time to market delays. Finally, these enclosures can actually redirect electromagnetic energy causing further EMI problems.
Accordingly, there is a need in the art for a low-cost EMI shield that can be added to a product with a problem integrated circuit. Such a shield should be easy to design and produce. Such a shield should also be capable of being added to the device after the design of the product is substantially complete without requiring significant redesign of the product. This allows the shield to be added to the device design after prototypes have been constructed and EMI measurements taken on these prototypes.
The present invention provides a low-cost EMI shield that fits around an integrated circuit package to absorb electromagnetic energy and dissipate it as heat. The shield has a simple design and is not ohmically conductive so it may be added to designs with little or no re-design of the product.
The present invention reduces the EMI radiated from an integrated circuit package with a perimeter of material that presents a lossy interface to high-frequency electromagnetic currents and doesn""t significantly conduct low frequency and direct current (DC). This perimeter is fitted around an integrated circuit package such that the gap between a heat sink or other top conductor and the printed circuit board is completely closed by the lossy material. The position of the lossy material may be secured by a friction fit, adhesive, or self-locking or retaining features built into the lossy material.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.